This invention relates generally to face milling cutting tools and more particularly to a face milling cutting tool (hereinafter "cutter") comprising a cutter body and flat cutting bits or tips, each having around its periphery convexly curved cutting edges and so mounted on the cutter body that a specific axial relief angle is present between a front clearance face of each cutting tip and the face of a workpiece being machined while, simultaneously the radial rake angle of the cutting tip has a large negative value.
In general, a face milling cutter is used as a machining tool for machining a planar surface perpendicular to the rotating spindle axis of a machine. A cutter of this type has a cutter holder or body and a plurality of cutting bits or tips embedded in the cutter body. In one example of this type of face milling cutter, as disclosed in Japan Patent Publn. No. 35160/1977, a plurality of square tips are fixed by clamp screws to the working face of a cutter body. In the face milling cutter disclosed in this publication, the discharging of removed chips toward the periphery is facilitated by providing a radial rake angle of large negative value for each cutting tip.
However, since each cutting edge is linear and the rake face is also planar, the entire length of the linear cutting edge engages and disengages simultaneously with the workpiece, whereby chattering vibration tends to occur. Particularly in milling a difficult-to-machine material, such as stainless steel or a heat-resistant steel, the required cutting force is great, and the resulting chattering vibration gives rise to chipping or adhesive wear of the cutting edge. As a consequence, a good finished surface could not be obtained heretofore with a cutter of this type. Furthermore, it is difficult and expensive to grind with high precision each cutting edge of a cutting tip parallel to the mounting face of the cutter body. In the case where chamfered parts are provided at opposite ends of a cutting edge, the linear cutting edge becomes short, and the feed quantity is limited, whereby the machining efficiency drops.
Furthermore, a cutting tip having a curved cutting edge is shown in FIG. 7 of the above mentioned patent publication. However, since the thickness of the cutting tip in the direction of the impact of intermittent cutting and of the principal component of the resistance to cutting is small, there arises a serious problem in that the cutting edge is readily chipped during cutting, whereby this cutting tip is unsuitable for cutting steel materials and difficult-to-machine materials of high cutting resistance. Furthermore, since a stepped projection exists on the rake face of this cutting tip, the outward flow of removed chips is obstructed.
In still another example of a known face milling cutter, as disclosed in the specification of U.S. Pat. No. 3,670,380, a circular cutting bit or tip is fixed to the front working face of a cutter body. This cutting bit has a frustoconical shape, and its cutting edge is of circular shape projecting in the cutting direction. In order to increase the cutting depth, the bit is inclined by an angle .gamma. in the feed direction. Therefore, the point of maximum cutting depth of the bit is shifted in the direction of the cutter center and the cutting edge of that part is at a positive radial rake angle. The removed chips are sent inward and impair the finished surface. In the above cited specification, moreover, there is no disclosure of finishing cutting by an inclined cutting edge.
In still another U.S. Pat. No. 4,175,896, a ball end mill is disclosed. The cutting edge of this end mill is a convex cutting edge. However, the cutting edge of this end mill does not constitute a cutting edge which is inclined relative to the cutting direction and is incapable of planar cutting.